MYP Physics Planets and Satellites Notes

"What Counts As A Planet And A Satellite?"

1. A planet is a large natural object in space that moves around the Sun along a fixed path.
2. This path is not random; it follows a predictable route controlled by gravity.
3. Planets do not produce their own light but can be seen because they reflect light from the Sun.
4. The motion of planets helps scientists identify and classify them.

The IAU Definition Of A Planet (2006)

1. In 2006, the International Astronomical Union (IAU) defined a planet in our solar system as an object that:
   1. is not so massive that it starts fusion (otherwise it would become a star),
   2. orbits the Sun,
   3. is sufficiently massive that gravity makes it nearly round,
   4. has cleared the neighborhood around its orbit.
2. The last condition explains why Pluto, Ceres, and Eris are classified as dwarf planets rather than full planets: they share their orbital region with many other objects.

The Solar System Has Eight Planets With Very Different Properties

1. In the modern view, the solar system contains eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
2. They can be grouped into:
   1. Terrestrial (rocky) planets: Mercury, Venus, Earth, Mars (smaller, denser, solid surfaces)
   2. Giant planets: Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants)
3. A key pattern is that distances between planets increase strongly as you move outward.

Approximate Sizes And Distances

The table below summarizes typical distances from the Sun and planetary radii.

Orbits and Planetary Motion

How do planets move around the Sun?

1. Planets move along paths called orbits.
2. An orbit is usually elliptical, meaning it is slightly stretched rather than perfectly circular.